This invention relates to control techniques of the image quality of a decoded image and prediction techniques of the image quality of a decoded image when image compression is executed.
In recent years, images have been compressed for reducing the storage media capacity or the transmission time. In the description that follows, image coding is used in the same meaning as image compression.
As shown in FIG. 41, input images are compressed, transmitted, and stored in a system wherein image input machines such as scanners or image generation machines such as computers and image output machines such as printers are connected by a network. In recent years, images used in such a system have become high definition, colored, and a large capacity, thus it becomes important to raise the image compression ratio.
The image compression systems are classified into reversible and non-reversible systems. In the reversible compression system, if an image is decompressed after it is compressed, the original image can be restored completely. In the non-reversible compression system, a compression ratio higher than that in the reversible compression system can be expected, but if a compressed image is decompressed, it cannot completely be restored to the original image, causing degradation of the image quality.
Generally, to execute image compression, inversely proportional relationship would exist between the compression ratio and the image quality under the same coding conditions, because the non-reversible compression system realizes a high compression ratio by discarding information considered to be hard to affect the sight sense in an image. Thus, if the compression ratio is small, a small amount of information is discarded and the image quality is good; if the compression ratio is enlarged, discarded information is increased and the image quality is degraded.
When image compression is executed, it is desired to maintain a predetermined image quality and realize a high compression ratio as much as possible. That is, the coded image quality needs to be controlled so as to become the maximum permissible image quality.
Conventional non-reversible compression systems for controlling the coded image quality will be discussed below: